A mobile apparatus can be controlled to perform an operation as a function of a position of the mobile apparatus. Such operations can include modifying a surface over which the mobile apparatus is moved, ejecting a liquid onto a medium, printing an image, fabricating a device, or cutting a surface, for example. The accuracy to which the position of the mobile apparatus must be known depends upon the operation to be performed and the quality of the resulting output that is required. For example, the print quality of a sign that is to be viewed at a long distance does not require as high a degree of positional accuracy of printing as does a poster-sized print of a photographic image. In addition, the placement of different portions of an image that are separated by white space is not as critical of the placement of different portions of an image that are adjacent to each other and printed on separate printing swaths.
U.S. Pat. No. 6,116,707 discloses a robotic plotting system in which a printhead is placed on a substantially flat horizontal surface upon which a recording medium is placed. The robotic plotter printhead is directed across the medium by infrared, ultrasound or microwave signals that are transmitted to the printhead from the periphery of the recording medium. The printhead includes a motorized drive mechanism that propels it across the surface of the recording medium using control signals. U.S. Pat. No. 6,951,375 discloses a wheeled vehicle that includes motors, encoders, and an inkjet printhead for printing on a large surface area or walkway.
What is needed is a simple, reliable and accurate way of determining the position of a mobile apparatus for performing an operation, such as printing, as a function of the position of the mobile apparatus. It is also advantageous for positioning reference structure to include a built-in distance measurement reference.